A trip into New York City’s sewers involves a number of well-known hazards: ninja turtles, giant albino alligators, mole people, and of course, rivers of putrid, slimy grease. But now we have to add gold-rush-style grizzled prospectors, industrious methheads and other precious-metal fanatics to the list of unsavory characters one might encounter in the city’s miles of tunnels.

A recent University of Arizona study found that each year, the sewer sludge from a city of a million people contains about $13 million in metals like gold, silver and platinum. If the research team’s findings are scalable, that would mean over $100 million worth of annual squandered bling in a city the size of NYC. Storm run off, industrial byproducts, and whatever else people flush down the toilet all ultimately contribute to the fortune hidden in our underground poop reservoirs. And while extracting these metals might seem like an unworkable chore, doing so could easily offset some of the high waste-processing costs incurred by bigger cities. Science magazine reports:

And it’s possible that precious metals are just the tip of the excrement iceberg.

“A small number of sewage plants are removing phosphorous and nitrogen, which can be sold as fertilizer. A Swedish treatment plant is testing the feasibility of making bioplastics from wastewater. A model sewage incinerator that generates electricity and drinking water was just promoted by the Bill & Melinda Gates Foundation, which helped fund its construction.”

Currently, a little more than half of our sewage sludge is processed into fertilizer, with the rest headed for landfills and incinerators. But lately, the lingering presence of toxic residues, chemicals and germs has caused municipalities and agronomists to rethink the way we deploy our sludge. It will still probably be some time before mining muck becomes a regular practice; these valuable substances are usually present in microscopic amounts, and extracting them can be less than cost-efficient. But as more cities experiment with leveraging their sewage for its secret treasures, the process should become increasingly more cost efficient, with scientists working toward a future where sludge is an important commodity rather than just a disgusting byproduct.

Two innovative companies, Blackgold Biofuels and FogBusters, are unlocking the power of fat, oil, and grease (more pleasingly referred to in aggregate as "FOG"). The two startups are the runners-up in Imagine H20's Water-Energy Nexus Prize, which was created to attract and accelerate new water businesses with energy-saving innovation. Prizes were awarded earlier this week.

Blackgold Biofuels is operating FOG-to-Fuels, a pilot program to turn FOG into biodiesel at San Francisco's Wastewater Treatment Facility. FOG builds up in sewer pipes much like cholesterol does in an artery, and that backup causes some 10 billion gallons of untreated sewage flow each year (yuck). Blackgold's process, says CEO and co-founder Emily Bockian Landsburg, not only reduces the amount of energy needed to treat FOG but also results in "an environmentally friendly fuel that can be used onsite or sold for profit." Conversion to biodiesel offers the highest and best use for this material.

FogBusters has developed a patented technology that works in conjunction with existing systems to actually remove FOG from wastewater without the use of chemicals. The reasons why this is significant include the fact that there is currently so much FOG sludge it needs to be transported by truck to landfills, where it takes forever to break down. Removing FOG from the wastewater equation is, says FogBusters CEO Bradley Mart, "the first big breakthrough in oil/water separation in 25 years."